Limitations of rupture forecasting exposed by instantaneously triggered earthquake doublet

Abstract

Earthquake hazard assessments and rupture forecasts are based on the potential length of seismic rupture and whether or not slip is arrested at fault segment boundaries. Such forecasts do not generally consider that one earthquake can trigger a second large event, near-instantaneously, at distances greater than a few kilometers. Here we present a geodetic and seismological analysis of a magnitude 7.1 intra-continental earthquake that occurred in Pakistan in 1997. We find that the earthquake, rather than a single event as hitherto assumed, was in fact an earthquake doublet: initial rupture on a shallow, blind 2 reverse fault was followed just 19 seconds later by a second rupture on a separate reverse fault 50 km away. Slip on the second fault increased the total seismic moment by half, and doubled both the combined event duration and the area of maximum ground shaking. We infer that static Coulomb stresses at the initiation location of the second earthquake were probably reduced as a result of the first. Instead, we suggest that a dynamic triggering mechanism is likely, although the responsible seismic wave phase is unclear. Our results expose a flaw in earthquake rupture forecasts that disregard cascading, multiple-fault ruptures of this type.Continental earthquakes typically rupture diffuse systems of shallow fault segments, delineated by bends, step-overs, gaps, and terminations. The largest events generally involve slip on multiple segments, and whether or not rupture is arrested by these boundaries can determine the difference between a moderate earthquake and a potentially devastating one.Compilations of historical surface ruptures suggest that boundary offsets of ~5 km are sufficient to halt earthquakes, regardless of the total rupture length 1,2 . This value is incorporated into modern, fault-based earthquake rupture forecasts such as the UCERF3 model for California 3,4 , whose goals include anticipating the maximum possible rupture length and magnitude of future earthquakes within known fault systems.However, if earthquakes could rapidly trigger failure of neighbouring faults or fault segments, at distances larger than ~5 km, then such scenario planning could be missing an important class of cascading, multiple-fault rupture. Here we exploit the combination of spatial 3 information captured by satellite deformation measurements and timing information of successive fault ruptures from seismology, to reveal how near-instantaneous, probably dynamic triggering may lead to sequential rupture of multiple large earthquakes separated by distances of 10s of kilometers.The destructive Harnai earthquake occurred on 27 February 1997 at 21:08 UTC (02:08 on 28February, local time) in the western Sulaiman mountains of Pakistan 5 (Figure 1a). Published source catalogues ascribe it a single, largely (85% 99%) double-couple focal mechanism with gentle ~N-dipping and steep ~S-dipping nodal planes and a moment magnitude M w of 7.0 7.1 (Supplementary Table 1 The south-eastern fringe ellipse is caused by slip on another NE-di...

Complex multifault rupture during the 2016 M w 7.8 Kaikōura earthquake, New Zealand

“…However, identifying which of the faults is the keystone fault will require further work. Regardless of the rupture mechanism, considering the incompleteness of many global fault databases, which typically only show surface faults, these observations highlight the need to account for larger jumps in hazard models which may be accommodated by unmapped faults or dynamic triggering (5,6,35).…”

“…Variations in stress levels along a rupture and geometric complexities, such as fault step overs, are thought to be a primary control of final rupture length (1,2). While numerical models and field observations suggests that fault step overs of more than 4-5 km can halt a ruptures' propagation (1,3,4), nearinstantaneous triggering over distances of more than 50 km has been documented (5,6). Furthermore, recent observations indicate that fault networks with both optimally oriented and misoriented faults can rupture during a single earthquake (7,8).…”

The role of space-based observation in understanding and responding to active tectonics and earthquakes

“…In order both to fully constrain and advance understanding of many tectonic problems, it is often necessary to combine the suite of techniques listed above. Seismological observations are global in coverage for moderate and larger earthquakes and can provide the high-resolution temporal sampling necessary to look at the rapid evolution of an earthquake rupture over seconds, which is not possible using static measurements made from relatively infrequent satellite passes20. Seismology has been the principal tool for the study of earthquakes for most of the 20th century, particularly in remote areas where field observations are not easily made.…”

“…The modelled observations of slip in the near-surface can then be compared with field observations of discrete mapped surface offsets (green circles). Determining the geometry of faulting and its relationship with surface geomorphology is important for interpreting the surface fault expression52 and understanding the segmentation of rupture20 for estimating potential seismic hazard. Establishing the extent of slip is needed for determining which portion of the fault failed in the earthquake, and also which did not and could fail in future841.…”

Source Parameters of the 2003–2004 Bange Earthquake Sequence, Central Tibet, China, Estimated from InSAR Data

“…To test whether more recent earthquakes in the Bange earthquake sequence may have been triggered by previous ones, we calculate the progression of the Coulomb stress changes by using the PSGRN/PSCMP code based on the distributed slip source [35] (e.g., [36]). First, we calculate the static (Figure 16b).…”